ISSN 1026-0269
FROGLOG Newsletter of the Declining Amphibian Populations Task Force
The Effects of Habitat Modification on Spotted Frogs in Yellowstone National Park By Debra A. Patla A report on work carried out under the DAPTF Seed Grant Programme. This DAPTF grant supported a study replicating and comparing Frederick B. Turner's investigation (1953-1955) of a spotted frog population (formerly Rana pretiosa, now Rana luteiventris) in Yellowstone National Park, Wyoming, U.S.A. Turner's study documented population size and thoroughly described seasonal activity patterns and spatial relationships in the 1950s. Our reinvestigation was stimulated by Fred Turner's visit to the study area in 1991, during which he observed that spotted frogs appeared to be greatly reduced in numbers and that development had caused obvious changes in portions of the study area. In using historical information as a research tool, we followed one of the main recommendations of the National Research Council's Board on Biology workshop on declining amphibians (February 1990 in Irvine, CA). Our objectives were to verify and document the suspected decline of the population inhabiting the study area; to determine how ecological characteristics, particularly spatial relationships, have changed; and to determine the causes of the decline. Our approach was to replicate Turner's study and to compare the historical and current information and data sets. We hypothesized that adverse weather patterns or local habitat changes were the most likely causes of a decline. Employing methods similar to Turner's, we conducted intensive mark-recapture work through three consecutive summers (1993-1995) in the 28-hectare main study area. We
mapped all capture-recapture locations and used small radio transmitters to track the movements of several adult frogs. We compared demographic characteristics, spatial relationships, weather patterns and environmental conditions during the 1950s and the 1990s. Comparisons were feasible because most of the important habitat features identified by Turner (e.g., streams and pools) are still obvious in the 1990s. However, two major development projects have occurred since the 1950s. The Park highway was rerouted in the 1970s, with its new path cutting through the middle of the study area and in the 1980s, an underground water pumping system was installed at the headwaters of the main drainage in the study area. We found that the spotted frog population occupying the main study area has declined substantially, dropping from about 1200-1900 frogs in the 1950s to about 200-400 frogs in the 1990s. Comparing spatial characteristics of the frog populations in the 1950s and the 1990s, we found changes that appear to be independent of the reduction in population size. The relative distribution of frogs in the study area has changed, with the frog population clumped in some areas and absent from others, in comparison to the 1950s. The patterns of frog movements among upland areas, breeding sites, and permanent water that Turner discerned in the 1950s are restricted to fewer areas in the 1990s; only 29% of the former movement patterns remain. Three main activity ranges of the 1950s, consisting of breeding, foraging, and overwintering areas, have been reduced to one main pattern in the 1990s. The 1990s study was not preceded by abnormal or adverse weather patterns that might explain these changes in the frog population. Habitat conditions have deteriorated since the 1950s; former overwintering, foraging, and breeding areas have been reduced in number
August 1998, Number 28. and size, and a major frog migration route between summer and winter habitat was bisected by the new highway. While most of the habitat degradation appears to be the result of human activities, potentially natural shrinkage of ephemeral pools used by frogs for breeding has also occurred. Although the precise cause of the population decline cannot be inferred from the then-and-now data sets available to us, habitat degradation appears to provide the mostly likely explanation. The reconfiguration of frog distribution and activity patterns relative to the 1950s indicates changes in the quality, relative importance, and connectivity of habitat features. Habitat fragmentation caused by the new highway, downstream effects of water pumping at the headwater springs, and the recent cessation of breeding at one of the remaining former breeding sites appear to be the major factors that limit habitat and minimize the potential for an eventual population recovery. The following generalizations based on our case study may be useful to the study of amphibian population declines: 1) Former herpetological studies can provide valuable information that may be used to decipher factors contributing to the persistence and decline of populations. 2) Studies that are limited to amphibian breeding sites may miss important relationships that also affect population size and persistence e.g., seasonal movements to foraging areas and wintering sites. 3) Raw data from current studies, including daily field notes, should be safely archived where other researchers can access them. Future researchers may rely on these data for comparative studies; published summaries of results may not be sufficient to answer the questions of future researchers or to perform statistical tests.
The World Conservation Union (IUCN)/Species Survival Commission (SSC) The Open University • The World Congress of Herpetology • The Smithsonian Institution